Archive for February, 2005|Monthly archive page

…and another abstract

Today was the abstract deadline for CGU, a smaller (but friendly and low-key) meeting. I put an abstract in on behalf of an undergrad working with me:

Mantle Fabric and Lithospheric Thickness Beneath the Superior Province

S.-K. Miong and A.W. Frederiksen
Department of Geological Sciences, University of Manitoba

The Superior Province, the largest Archean craton in the Canadian Shield, represents an ideal laboratory for understanding the nature and development of cratonic lithosphere. Ontario spans a major portion of the Superior, and, under the auspices of the POLARIS and FEDNOR projects, is in the process of being instrumented with broadband seismometers on a large scale. We present the result of SKS splitting analyses for FEDNOR and CNSN stations spanning the breadth of Ontario, covering the Eastern and Western Superior Province. The Western Superior exhibits very large SKS splits (averaging 1.4 seconds) with a consistent ENE fast direction (averaging 69 degrees azimuth). In the Eastern Superior, the fast directions are much more variable (ranging from east to northeast), with smaller split times averaging 0.8 second. In the Western Superior, the split times align closely with both the current direction of absolute plate motion and the orientation of structural belts in the crust; we therefore interpret the strong splits in this region to represent a combination of lithospheric and asthenospheric fabric sharing a common alignment. In the east, the fast directions show appreciable scatter around the direction of plate motion, though there is general agreement; given the weaker split times in this area, we take the variability to reflect a weaker and more inconsistent lithospheric fabric, since the asthenospheric fabric should vary little across the Superior. Results from other studies, including tomography, heat flow, and elastic plate thickness studies, suggest the possibility that the cratonic lithosphere may have been significantly reworked or thinned beneath the eastern portion of the Superior Province.

This one’s a little more definite because, well, the work’s mostly already done.


Spring AGU abstract

…because these abstract deadlines always manage to sneak up on me:

Inversion of the teleseismic P coda for lithospheric structure: Examples from Ontario and California

A.W. Frederiksen and J. Zhang, University of Manitoba
J. Revenaugh, University of Minnesota

The coda of the teleseismic P wave has become one of the most powerful tools for unravelling fine-scale receiver-side structure, using both single stations and sparse or dense arrays of seismometers. Determining structural information from the coda is an inverse problem that may be treated using either linear or nonlinear methods, depending on what ad hoc assumptions are made about the nature of the coda waves and the structures that generate them. We will review some of the principal methods used in coda imaging and inversion, and examine two methods in greater detail: a non-linear search algorithm applied to single-station data in the presence of anisotropy and dip, and a linearized tomographic inversion of scattered-wave energy in the coda. Examples of applying these methods to detect thinly laminated mantle anisotropy beneath southern Ontario and features correlated with seismicity in California will be given.

It’s kind of vague, because I’m not sure what’s going to make it into the talk at this stage — but, since Partha talked me into co-chairing a session with him, I might as well take the opportunity to try to publicize my scattering-tomography method a bit better. The chairing part of the equation will be a first for me, as well — better not nod off during any talks…


My colleague, Ian Ferguson, just had the somewhat unique experience of reading his own obituary. I’m sure the Leading Edge will be rather embarassed when they figure it out. In the meantime, I can reassure anyone who knows Ian that if he’s dead, he’s doing a remarkable job of not showing it. I think a dryly witty letter to the editor would be called for — I’ll pass on any good suggestions to Ian.

But instead of being swayed by either side, we at UDN, Inc. have found a theory that effectively
merges the strengths of the two theories without the weaknesses.  The intelligent design people
say there are too many holes in the fossil record, and that evolution is only a theory; the
scientists say there’s not enough evidence of intelligent design.  So we say, instead, that
life has indeed been designed, just not very well.

All hail Unintelligent Design!

Oddities in journal editing

I’m more lax about reading journals than I really should be, but I do subscribe to the paper version of JGR. I received my copy of the November issue (scientific journals have a tendency to be late) this morning, and on a quick flip through it , I noticed the following articles:

Hearn, Thomas M.; Wang, Suyun; Ni, James F.; Xu, Zhonghuai; Yu, Yanxiang; Zhang, Xiaodong, "Uppermost mantle velocities beneath China and surrounding regions"

Liang, Chuntao; Song, Xiaodong; Huang, Jinli, "Tomographic inversion of Pn travel times in China"

So what’s odd about that? Well, they’re almost the same study — two independent groups of researchers, applying more or less the same method to more or less the same data set. And, of course, getting remarkably similar results. So how’d they end up in the same issue? I have no idea, but my best guess would be that both were in the pipe at the same time, and rather than try to determine priority, the editor decided to slot them in together.

I must say, it’s nice to see a repeatable experiment in geophysics.